DESCRIPTION (Principal Investigator's Abstract): The objective of this proposal is to study the way lean and obese men and women adjust energy expenditure and the fuel mixture oxidized by the body in response to changes in diet composition. A stable body weight is the result of equality between total energy ingested and total energy expended as well as between total intake and oxidation of each macronutrient - protein, carbohydrate and fat. When diet composition is changed, the body must adjust substrate oxidation in order to reestablish a new equilibrium where oxidation of each macronutrient equals intake of that nutrient. The investigators propose that lean humans differ from obese humans and men differ from women in either the type and/or rapidity of mechanisms used to reestablished energy/substrate equilibrium following a change in diet composition. Further, the investigators propose such differences would lead to different levels of body weight and body fat mass at the point the new equilibrium is reached. The development of obesity may be a necessary condition in some individuals for establishment of energy and substrate equilibrium. It may take several weeks to completely reach a new energy/substrate equilibrium after a major change in diet composition. It would be difficult to conduct the necessary long-term studies, carefully controlling total food intake, to completely study such changes. The investigators have demonstrated that substantial changes in substrate oxidation occur within 7 days of a change in diet composition. They propose that studying the responses of lean and obese men and women to a 7-day change in diet composition will indicate differences in the types of mechanisms used and/or the rapidity with which move toward reestablishment of energy/substrate equilibrium. The investigators use a new whole-room indirect calorimeter to study daily energy expenditure and daily substrate oxidation changes in response to alterations in the carbohydrate and fat content of the diets. The calorimeter provides continuous measures of energy expenditure and oxidation rates of carbohydrate and fat (after determining protein oxidation). In each of the 4 proposed studies, lean and obese men and women will be brought into the Clinical Research Center (CRC) and fed a high fat and a high carbohydrate diet for 7 days each (with a 1 month period between diets). Separate studies will involve variables such as whether total food intake is limited or ad libitum and whether or not the subjects engage in daily aerobic exercise. The goal of this work is to determine if the composition of food eaten plays a major role in body weight regulation and whether obese individuals are more prone to increase fat mass in response to increases in the fat content of the diet than are lean individuals. Additionally, the investigators will follow-up on preliminary data suggesting men have a greater capacity than women to maintain a high fat oxidation when placed on a high carbohydrate diet. The results of the proposed studies should be directly related to understanding how obesity develops and how it can best be prevented and/or treated.